This invention relates generally to fuses and, more particularly, to fuses with a fuse state indicator.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. A fusible link is connected between the fuse terminals, so that when electrical current flowing through the fuse exceeds a predetermined limit, the fusible link melts and opens the circuit through the fuse to prevent electrical component damage.
Fuse indicators have been developed for various types of fuses to facilitate identification of inoperable fuses due to an opened fuse link. One type of fuse state indicator includes conductive thin metallic films extending on an outer surface of a insulative fuse body. See, for example, U.S. Pat. Nos. 6,459,357, 6,456,189, 6,292,087, and 5,994,993. In such fuses, one or more layers of metallic film extend on the outer surface of an insulative fuse body between conductive end caps or terminals, sometimes referred to as ferrules, that are attached to the opposite ends of the fuse body with a crimping operation. A conductive path is therefore established through the metallic films for fuse state indication, and the conductive path through the metallic film is established in parallel with the primary fuse link. When the primary fuse link operates to open the electrical circuit therethrough, current flows through the thin metallic films which visibly indicate the operational state of the fuse. Opened fuses may therefore be readily identified and replaced.
Termination of the thin metallic films to the ferrules of the fuse has proven problematic. It is difficult to attach the ferrules to the indicator without damaging the indicator. U.S. Pat. Nos. 6,459,357, 6,292,087 and 5,994,993 each include conductive leads to connect the indicator to the respective ferrules. The conductive leads complicate the construction and manufacture of the fuse, as well as introduce potential reliability issues in operation.
Moreover, undesirable electrical arcing from ferrule-to-ferrule may occur when known fuse state indicators including thin metallic films operate to indicate an opened fuse. Especially in smaller fuses having a reduced longitudinal distance or separation between the ferrules of the fuse (e.g., fuses having an axial length of about two inches or less), the tendency for arcing on the exterior of the fuse body between the ferrules has been demonstrated. As the ferrules are crimped to the fuse body, initial electrical contact between the metallic film indicators and the ferrules is made at the edges of the ferrules where they are crimped to the fuse body. When the fuse state indicator operates, and especially when the indicator opens near the ferrules, metallic vapors are generated at the contact points between the ferrules and the indicator. Uncontrolled discharge of metallic vapor is believed to contribute to ferrule-to-ferrule arcing.
U.S. Pat. No. 6,456,189 describes conductive rings attached to a fuse body in electrical connection with the fuse state indicator, and conductive ferrules are attached over the conductive rings to complete a conductive path through the indicator. The construction appears to at least partially obviate conditions that create electrical arcing between the ferrules, but the conductive rings introduce additional components and associated cost to the fuse construction.